Method of producing gypsum plaster



NOV. 4, 1952 ,5, A, HOGGATT 2,616,789

v METHOD OF' PRODUCING GYPSUM PLASTER Fle March 19, `1951 Patented Nov. 4, 1952 METHOD F PRODUCING'GYPSUM PLASTER 'Gilbert A. Hoggatt, Park Ridge, Ill., assignor to Certain-Teed Products Corporation, Ardmore, Pa., a corporation of Maryland Application March 19, 1951, Serial No. 216,434

8 Claims.

A:setting upon mixture with Water.

` "This application is a oontinuation-in-part of "'applicant's copending application Serial No. l526,005, iiled March 11, 1944, now forfeited.

In the production of calcium sulphate heinid dydrate in the form of plasters, gypsum, `which consists predominantly of calcium sulphate di- A"hydrate, ordinarily is heated in a dry state to drive off the water of crystallization to reduce the lcombined Waterfrom two molecules to one-half molecule (CaSOrl/ZHzO) per mole- `cule of calcium sulphate. In common manufac- '1 turing practice, gypsum rock is mined, reduced tto suitable particle size, and calcined in a kettle or a rotary calciner, heated to raise'the temperature of the mass of gypsum to that at which the Water of crystallization is driven off.

Calcined gypsum as ordinarily produced. in the formof gypsum plasters or plaster of Paris has the capacity to .take up Water oi crystallization l`and to set. The time required for setting of a mortar or slurry made of a mixture of calcined lgypsum and water, 'the amount of water required to produce 'a given consistency of the mortar or slurry, the plasticity and other qualities of the .mortar `or slurry, and the strength, density and hardness and other qualities or the cast made therefrom Vary with different ravrv materials and Withthe conditions of calcination. This inven- 'tion is particularly concerned with that quality vof the calcium sulphate hemihydrate which determines the amount of water which is required "to produce a given consistency in a mixture of kthe .hemihydrate with water. Calcined gypsum Yas ordinarilyproduced in a calcining kettle requires a relatively large amount of Water to pro- .ducefa `given consistency, for example, the so- `called pouring consistencyfat which the mixture may be poured, for example, into va mold.

,The amount of water used in mixing has considerable effect upon the strength and density ofthe I.set product. It is Well recognized that reduction .inthe amount of Water used to produce a mortar or slurry of a calcium sulphate hemihydrate .or

a calcined gypsumplaster increases the strength .as wellas the density of theresulting cast or set product. The invention is concerned with a process which'makes itpossible to use, with the product of .this process, a greatly reduced amount ofwaterfto .produce `a given consistency `as compared with vordinary calcined gypsum plasters .and thereby .to secure .in the set product high density;v hardness .and strength.

-terials 2 Heretofore, `it has Vbeen proposed to produce so-called calcined gypsum or calcium sulphate -hemihydrate having a low Water carrying capacity, requiring only a small amount of Water to produce a given consistency. In the patent to Randel and Dailey 1,901,051, March 14, 1933,"is disclosed a `process of calcining gypsum und-er steam pressure of limited range above atmospheric pressure to produce what is termed alpha gypsum having a crystalline structure charac.- terized by crystals which are non-porous, short, relatively thick rods or prisms. In the patent to Schoch, 1,989,712, of February 5, 1935, there is disclosed a process of heating gypsum in a solution of magnesium sulphate at the boiling temperature of the solution preferably secured by raising the pressure of boiling in a closed vessel above atmospheric pressure. In a paper entitled Production of Gypsum Plaster by Wet Methods by E. P. Schoch and William A. Cunningham, Transactions, volume 3'7 of 1941, American Institute of Chemical Engineers, reference is'made to both said patents. It is stated in this 'paper that heating calcium sulphate dihydrate in la magnesium sulphate solution produced crystals which were stort and rodlike. This 'paper also states that such crystals could not be produced in solutions of sodium chloride and calcium chloride. It indicates for the product produced'by heating in magnesium sulphate a Water-carrying capacity of similar degree to that disclosed in the Randel .and Dailey patent.

In this paper the term Water-carrying capacity is stated to be theamount of Water required to make a slurry of normal consistency. While the term normal consistency is not defined, it is implied that this term has the meaning as ',dened bythe American Society of Testing Ma- In the Randel .and Dailey Patent No. 1,901,051 normal consistency is defined as that amount of water in cubic centimeters or grams, which, when mixed with grams of dry stucco .will produce a mix of such consistency that it Will just pour from a cup.

This denition, however, is not in agreement with the definition of normal consistency given by the American Society for Testing Materials. The terms testing consistency and .normal consistency are delined as equivalent .tto each other by the American Societyfor Testing Materials under the designations C-26-33 and C-26-42 for the Standard Method for Testing Gypsum and Gypsum Products. For the purposes of the present invention, for the most part it is suiiicient to indicate the results obtained as measured by pouring consistency" which substantially is the same as the normal consistency" defined in the Randel and Dailey patent.

Contrary7 to the experience described in .the

vdisclosures of the-.Schoch Patent No. .1,989,712

, temperatures.

and in said paper by Schoch and Cunningham, in accordance with the present invention conversion of calcium sulphate dihydrate to calcium sulphate hemihydrate to form a settable plaster of low water-carrying capacity may be carried on in solutions of metallic salts such as the halides, nitrates and sulphates of sodium, potassium, magnesium, calcium, caesium, zinc, copper and ammonia and including specifically sodium chloride and calcium chloride stated by Schoch and Cunningham to be unsuitable for the production of acceptable low water-carrying plaster. Moreover, with the metallic salts used in the present invention crystals of the type which may be produced by the process of the Randel and Dailey Patent No. 1,901,051 may be produced. The in- Yvention is concerned with a process of producing calcium sulphate hemihydrate by heating calcium sulphate dihydrate in a water solution of a vapor pressure depressant, particularly a metallic salt or mixtures thereof, at a pressure not in excess of atmospheric pressure. The product produced by this process will have a water carrying capacity of a similar low degree to that of the product produced by the Randel and Dailey patent, or lower. From the product of the process of the invention hard casts of high strength and high density may be secured when the slurry is mixed at pouring consistency, which is a consistency substantially the same as that commonly used in moulding or casting operations.

`It is well known that the vapor pressure of a solution of a metallic salt in Water is lower than the vapor pressure of the water alone at the same temperature. Correspondingly, the boiling point of the Vsolution is greater than that of water at the same pressure. In Bulletin No. 33 of the Bureau of Soils of the United States Department of Agriculture, under title Calcium Sulphate in Aqueous Solutions by Cameron and Bell, and in Technical Paper 625 of the Bureau of Mines of the United States Department of the Interior, Thermodynamic Properties of Gypsum and its Dehydration Products. by Kelley, Southard and Anderson, referring to the work of Van-t Hoff, data and curves are given showing the vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at various .There are alsogiven similar data and .curves for water as well as solutions of sodium chloride and magnesium chloride and data for solutions of calcium chloride. From other sources it is possible to plot curves showing the relation of the vapor pressure to temperature for solutions of various salts at various concentrations of the salts in the water. As shown in the Bulletin No. 33 of the United States Bureau of Soils, the intersection of the vapor pressuretemperature curve of gypsumrwith that of the solution of the salt indicates the temperature and the corresponding vapor pressure at whichA as the temperature of the Ysolution is increased conversion from calcium sulphate dihydrate to calcium sulphate hemihydrate starts. For the purposes ofV description of the invention, the temperature corresponding to this point of intersection will be referred to as the inversion temperature.

The invention, in common with the earlier proposals, preferably effects removal of the water of crystallization by heating calcium sulphate dihydrate or gypsum in a solution of a metallic salt. It proposes to carry out this heating at atmospheric pressure. It is a particular feature of the discovery with which the invention is concerned that calcium sulphate hemihydrate in the form of thick, stubby rod-like crystals and having a low water carrying capacity may be secured by controlling within certain limits the diierence between the vapor pressure of the solution in which the calcium sulphate dihydrate is immersed and the vapor pressure of the gypsum at the temperature corresponding to said vapor pressure of the solution. That is to say, the vapor pressure difference measured at any temperature lying above the inversion temperature of the gypsum in a given solution must be maintained within said limits. The vapor pressure diierential between the two curves is a measure of the speed of conversion of the calcium sulphate dihydrate to the calcium sulphate hemihydrate. It `has been found that if this diierence of vapor pressure is too great the crystals produced will not be of the thick, stubby, rod-like form and the product will not have a water carrying capacity of markedly low degree. If, on the other hand, the vapor pressure difference is too small, the conversion takes place so slowly that the process will not be commercially practical, Contrary to the experience of Schoch and Cunningham who describe crystals which are long and needle-like when produced in solutions other than magnesium sulphate, it is possible when controlling the conditions, and particularly the vapor pressure difference above mentioned, in accordance with the invention to produce the thick, stubby, rodlike crystals and a product of loW Water carrying capacity and the other qualities above referred to.

The invention may utilize as vapor pressure depressants salts of the alkali metals and of the ammonium radical and of the alkali earth metals. By controlling the degree of concentration of such salt in the water the requisite difference between the vapor pressure of the gypsum and the vapor pressure of said solution may be obtained. Since for each degree of concentration the vapor pressure of the solution is determined by the temperature of the solution, it becomes possible in a series of curves of the vapor pressure of the solutions which are plotted to the same scale as the vapor pressure-temperature curve for gypsum to show the intersections of theseV solution curves with the gypsum curve and thus determine the vapor pressure and the corresponding temperature at which conversion of the dihydrate to the hemihydrate submerged in the solution will start, i. e., the inversion temperature. The minimum dierence determined in accordance with the proposals of the invention, as above referred to, between the vapor pressure of the gypsum and that of the solution, that is, a minimum excess of the former with respect to the latter, may be gauged for each of these solution curves. Correspondingly, the maximum allowable diiierence between the vapor pressure of the gypsum and that of the solution in which the gypsum is immersed also may be gauged for each of these solution curves. In the latter instance, however, this pressure diierence in some cases withinv the scope of the invention may be between the vapor pressure of the solution at the boiling temperature of the solution at atmospheric pressure and the vapor pressure of the gypsum at the same temperature.

While in the work of Van t Hoff, above referred to, the temperature designated by Van t Hoff as the inversion temperature and represented by the intersection of the vapor pressure-temperature curves of water and of calcium sulphate dihydrate in contact with calcium sulphate hemi- Technical-Paper; 6,25 of the f-iureauof"` Mines this vintersection :is lindicated. as :foccurring'ffat about y100 f-C. (2122" 5Fl) "This ffis the theoretical :point "ati Which-a :changel in the .amount of :iwater ',of crystallization occurswi-nfpassing :either from :the -dihydrate to: `the hemihydrateor" the reverse. It `may sbewconsidered :that .calcium .f-su'lphate di-hy- *dr-ate; gypsurnf'- generally `in accordancevwit-h .:the 4ideasexpressedby Wan "'lt Hoff, wcannot fexistiin `stable'condition1in contactwvith "calciumzsulp'hate inomi-hydrate I.fat-a ltemperature higher than that `represented by theintersectionfof -thefcurves-that `iis;"the:inversionltemperature liForf'thenpurposes 'of the invention, j'i-however, 'this ntersection .iis vconsidered to occur:l atm-2 fling-i760: rn-m. of mercury =.vapor Apressure =in :water -or fin a :saturated Watervapor `in `accordance with f'sai'd nPlle'chnical "Paper625- Allnhapplying- (these considerationsiinfthefpresent ofzthe :water 1in 'which the salt is dissolved. For comparison, also, on thisset ,of curves is shown the vapor:pressure-temperaturecurve for water. 'Allof :these-curves are carried to the boiling tem- 'peraturev of the solution at; normal atmospheric pressurezor'to .a temperature below boiling temperature which provides the maximum vapor pressure difference of 1000 mm.

The Vapor pressureftemperature curve A for gypsum is based on data obtained from the Technical Paper 625 of the Bureau of Mines for the portion of curveA, up to and 'including the intersection with the vapor pressure curve for y water, vthis intersection occurringat 212 F., as

above stated, For extrapolation of this curve above 212 F., the vapor pressure-temperature :curve of gypsum up to. .2l2 F.,asshown insaid gpaper, was plotted to logarithm scale. While this logarithmic ,curve is not exactlya 'straight invention it #is iffound, ihowever, Tthat by rextrarturef-Z 1211Fgit11is possible V:to -fuse'the :vapor: presfsure--fdiiterences `-de'atermined vbetween --Ythe--extra I'polated curve and the vapor pressure lcurves of Ethe "solutions 'to "arrive fat" the minimum 'vapor pressure differenceandfthe maximum Vvaporfpresv sure Vdifference `which substantially f will f establish ftherange of these: vapor-pressure 'differences and,

thereforeffor each solution, therange-'of tem-j' peratures at which conversion from the dihydrate tofthehemihydrate'must befcarried'foniin order to `v`secure -ftheidesiredL plaster. YUontrolfthus based on kextrapolation `of they gypsum curveisifoundto'be i f effective Vtoy secure Ithe -f-desire'd =product,-notwith standing the factthat the pressure difference so determined `may-vv or may -not-represent a differ- YVence between.'actually `exi-sting vapor pressures of gypsum fand voi the'A solution. *In accordance `with 'these methodsf'of determining ithe pressure diierence," it v)has rbeen"ifou-ndfthat said v-range of vapor pressure y*difference substantially 4lies VAbe;- -tween 'l5`0'-mm. -`and *1000 -mm. -of ^mercury- *ln View of this discovery-'for fthe' production of dcrystais of #calcumfsulphate *hem'ihydrate Of'thety'pe abovevreferrn-'d nto, it :impossible whenausingivaiif- Jous inorganic ysalts selectedffrom thefgroup consisting cfsalts of -the alkali-metals and of -theammonium radical and of the alkali earthmetalsj-to produce the desiredllowtwater carrying product, with its characteristiccrystals.

It has been ,lfound thatgsatisfactory:resultsare obtainediloyy using as the `inorganic-:salt` any `of the following salts either alone or.inxadmixture,

-f one with another:

Ammonium .l chloride, ammonium"-bromide, ammonium iodide, ammonium nitrate, ammonium sulfate, fca1cium chloride, calcium bromide, calcium iodide, calcium nitrate, magnesium chloride, `magnesium bromide, magnesium iodide, Amagnesium nitrate, sodium chloride, sodium ,bro-

mide, -c-sodiumf iodide, sodium nitrate, potassium chloride,` potassium bromide, potassium iodide, potassium nitrate, caesium chloride, caesium niftratewcaesium sulphate, v-zinc chloride, --zinc fgbroamide,rznciodidaszincznitrate, zincisulfate, cupric chloride? :cupric ibromdepfcupric nitrate, cupric :sulfate .:Inxthe drawing is :shownfae se't -of cur-ves, such :Vas-dras :been #above 'mentioned ffor solutions Hof :calcium gchloride. "The fseveralnvapor k.pressuretemperature curi/estareY plotted V`for solutions fof :various l:concentrations of the salt in the water, -thezpercentage being expressed as the weight of .anhydrousrcalcium chlorideebasedon the weight line, the curvature isslight and uniform. The extrapolation, therefore, by graphically extending the curveat thesame curvature above 212"Fis sufficiently accuratefor the purposes of the present invention. It has been found that such extrapolation serves as lthe basis for determination of the measure of the pressure difference 'between'that 4of the gypsum and that of the solutionfin-which the gypsum is immersed.

Conversion of the calcium'sulphate dihydrate to calciumsulphate: hemihydrate to produce the typejof' product above referred to may be carried `out in `solutions -in which the selected 'salt or mixture of selected salts 'is dissolved in such amount that the `conversionV will take Vplace at va temperature which in some cases may be below thenormal inversion temperature of gypsum to hemih-ydrate in pure water, namely, 212 F., or inother cases may be above this temperature. The `minimum pressure difference of mm. ordinarily falls on ordinates in the curve which areybelow the normal inversion temperature inlwater, namely, 212 F., although in some cases with solutions of lower concentration this minimumpressure difference may be obtained at a temperature of above 212 F; which may be the boiling-temperatureof thesolu-tion. `The maxi- -mum pressure dierencepnamely, 1000 mm. of mercury, will fall on ordinates which are in excessof 212-F. Insome cases, however,with solutions of higher concentration this maximum 1pressure difference may be reached at a temperature which is less than the boiling temperature of the solution stt-atmosphericpressure. Withsuch solutions of high concentration the-vapor pressure difference may greatly exceed 1000 mm. yat Vthe boiling temperature of the solution'at atmospheric pressure. In such cases, therefore, it becomes necessary to limit the temperature to which the solution is raised so that it will not -reach"the1boi1ing temperature at atmospheric pressure 'in order lnot to exceed the vapor pressureidiierence of 1000mm. `On the other hand, V:tor 'solutions of intermediate concentration the solution may be' heated tothe boiling temperature without exceeding the maximum pressure difference of 1000 mm., although r`generally this :vapor pressure diierence will be substantially in excess of 150 mm.

Conveniently, however, the vprocess of jthe irl- Vventionmay be practiced at the boiling tempera-tures of fthe solution. Examination of the 'curves-*will show that "for calcium chloride, for example, the `pressure'diierence measured at the boiling temperature at atmospheric pressure, that is :betweeni-mm-andthe vapor pressure of the gypsum at the same temperature, for any solution within the range of 20% and 70% concentration will lie Within the range between 150 mm. and 1000 mm. of mercury, which according to the invention it has been found will insure the production of the calcium sulphate hemihydrate having` the desired crystal form and physical properties.

,A l Approximately saturated solution.

9 At 760 mm. barometric pressure.

In Table I, for various percentages of concentration of the calcium chloride in the solution, are shown the minimum and maximum operating temperatures at 760 mm. barometric pressure corresponding, respectively, to the minimum vapor pressure diierence of 150 mm. and the maximum Vapor pressure diierence, which for solutions of higher concentration may reach 1000 mm. Some of the solutions of lower concentration, however, as stated above boil at such a temperature that the pressure difference be tween the pressure at boiling temperature, namely, atmospheric pressure 760 mm. of mercury, and the Vapor pressure of the gypsum as indicated by curve A extrapolated, is less than 1000 mm. These values at boiling temperatures are shown in Table I in the last column.

In the curves the minimum pressure difference of 150 mm. for the 30% solution, for example, is, as indicated by the portion L of the ordinate, measured between the vapor pressure curve of the solution at this concentration and the vapor pressure curve of the gypsum. This ordinate will be found at the temperature of 213 F., as indicated above in Table I. For this 30% solution, also, the portion M of the ordinate at the boiling temperature, namely, 224 F., lying between the vapor pressure curve for this solution and the vapor pressure curve of the gypsum is found to have a value of 245 mm., also indicated in Table I. This represents the maximum pressure difference attainable for carrying on the process when using this 30% solution.

Similarly, for the 70% solution, the portion S of the ordinate at 181 F. which lies between the curve for this 70% solution and the gypsum curve has a value of 150 mm. For this 70% solution, at the temperature 248 F. the portion T of the ordinate lying between the curve for the solution at this concentration and the gypsum curve extrapolated has a value of about 880 mm. The solution boils at this temperature so that the full maximum diierence of 1000 mm. cannot be attained at atmospheric pressure. Table I shows the maximum vapor pressure difference attainable at boilingk for the various solutions Y' of various concentrations. It will be apparent that with solutions of higher concentration than 7 0% the maximum vapor pressure diierence of 8i 1000 mm. may be attained or exceeded. By controlling the temperatures of the solution in which the gypsum is being heated, however, to that shown in the table, the maximum pressure difference of 1000 mm. may be controlled in the solutions of higher concentration.

In order to carry out the process, therefore, a solution of given concentration may be chosen. From the curves or from the table the temperatures corresponding to the minimum and the maximum vapor pressure differences then may be found. The solution in which the gypsum is immersed then may be heated to a temperature controlled at a point lying in the range between said two temperatures so determined.` It has been found in-accordance with this method of control to secure the type of calcium sulphate hemihydrate above referred to, and in consideration of the element of time, desired low water carrying capacity of the plaster produced, and the strength, density and hardness of the cast made therefrom, that the concentration of the salt `in the solution and the temperature to which such solution may be heated may beso chosen that preferably the vapor pressure diierence lies substantially in the range between 170 mm. and 450 mm. of mercury. In the case of solutions of calcium chloride, the preferred concentration of the solution thus may lie in the range between about 22% and 48% at boiling temperature.

While it is possible to produce the desired product with'solutions which `provide a vapor pressure difference of less than mm. of mercury, the heating in such case must be carried on for such a period of time that the process becomes commercially impractical. For example, in a, 70% solution of calcium chloride the stubby, rodlike crystals were made by heating high purity gypsum rock which all passed a 1/2 screen, in the solution to a temperature, approximately 149 F., providing a vapor pressure difference of about 47 mm. but the time was about four days. In general, with the. salts ofthe group above disclosed asvapor pressure depressants suitable for the purposes of the invention, not substantially less than 15% of the salts of high solubility will be required to produce the product in any reasonable time.

Table II Vapor Boiling Approximate Pres. Dif- Pourm Temp., Conversion g Kind of Cr stals F. nflng Time, Hrs. Conslst y 217 110 Barely started in 8 hours. 220 3% hrs 45 almost all short,

' rodlike. 227 320 2% hrs 45 D0. 233 470 2% hrs 48 largely rodlike, few

l needles. 247 860 1% hrs 5l largely rodlike,

some needles. 1,440 1% hrs 57 largely needles. 289 2, 780 l/ hr 65 almost all needles.

In Table II are shown the results of carrying out the process of the invention in calcium chloride solutions at boiling at atmospheric pressure of approximately 750 mm. The vapor pressure diierences in this table are taken from the curves as the difference between the vapor pressure of the gypsum at the temperature indicated and the atmospheric pressure of 750 mm. After completing the conversion the product was washed substantially free of CaCl2, dried at a temperature not markedly below 212 F., and then aardige? grouiidliin a st'eelrdis'c attrition mill. The pour ingconsistency andlform of the'crystal were'th'en determined.` Ball milling the converted and ground product results in a further loweringfof the pouring consistency. For instance, a pouring consistency of 45 after disc grinding Was reduced in 30 minutes of ball milling to 40.

In Table II, for the maximum vapor pressure diieren'ces attainable'at boiling asA above mentioned are given the time in hours'in which .con-1 version takes place to thehemihydrate of thef type which itis the object ofthe invention to:` produce. These observationsl were made in converting gypsum which the pieces oriv particles Wereof such size that all passed through 1/2 meshl screen. In lTable II, also, are given` thef pouring consistencies of thei product thus' proofthe particles orf pieces of the gypsum, will lie roughly in the range between 30 %V and'45% of' the calcium chloride based on vthe weight of the water inwhich'y it isV dissolved. The pulverized or crushed gypsum Yrock is then immersed in the solution' andf'the solutionis heated untilit boils.

Boiling is. continued until the gypsum is substantially completely converted to the hemihydrate. The time4 required for this conversionl with thesolutionat. such concentration will be generally in the range' betweentwo and' onequarter to threehour's',` as indicated in Table No. II. The rate of conversion, however, is aiected by the size of the rock particles' or pieces. The outside off a: lum'pfor4 piecev oft rock is "first convertedgand`r conversion gradually progresses; in-

wardly'tc theicenterof the'lump or piece; If the duced" by heating in the calcium chloride soluf tions. In Vvthe right-hand column of Table II is indicatedcin generalA terms the type of crystal whichf, Wasfformel as seen under the microscope.v It'vvill-be noted-that Where the time was reduced below one andorre-half hours the crystals become; long 4 and eneed-lelike, and the pouringconsistency is above 51.` Thesepouringconsistencies were determined upon the plaster-groundm a fneness such that approximately 95% thereof passed `througha 100 mesh sieve.

As;V above -stated,-,however, the process preferably-isfcarried out-,at 4'the boiling ytemperature of -the solution.' Operation at the Ibeilingpoint makesunnecessary a temperature control to insure that the vapory pressure difference is not exceededY and, onf the otherhandJ that it is always-'maintained at suicient amount to insure conversion ofthe gypsum to the hemihydrate in the formy of the desired crystals.'l When the conversion is carried onL Wit-hv the solution at:V the boiling temperaturef, there is secured the advantage that the---water of crystallization which lis removed fromvv the gypsum and. thus tends to dilute the solution is oiiset to substantial degree bythe..evaporationof-the water from the-solution.` Depending upon the concentration ofthe solution-selected,moreor'less Water will berequired tomaintainthe desired concentratiom or additional amountsof -thesalt may be added from 4time to time *to insure that the concentration-rismaintained.

In connection with the operation ofthe process atboilingftemperatures. ofl the solutions,- it also shouldvbernoted that the speedV at which` the conversion takes place,` that is, .the time required to completely effect `change from calcium sulphate dihydrate-to calcium sulphate hem-ihydurate,- maybe Ycontrolledk or modiedby change in the concentration of the solution in which the gypsum is submerged While= maintaining ythe diierrence between the vapor pressure of the solution and the -Vapor pressure of the'gypsum withinthe` range 1150 min.V to 1000 mm. of mercury.

Asthe. preferredcondition of carrying on the process of dehydration of gypsumandre-crystallization to hemihydrate of` the type above referred to, a water solutionlofcalcium chloride is adjusted s'o'lthat the'boiling'tmperature of process is cariredl out',t the imipurities. in the salt completedfwithin two to four-` hours;` theshorter time? being attainable withA more nely' crushed raw material. y

Since. ther" Water of crystallization Which Yis drivenzofffromthegypsum mixes with the Water solvent', the-solution will-becomeV diluted unless,` inA carrying out Ithe processV at boiling temperae tures; thefevaporation equals or'exceeds the rate of *release'oflfthe "water of crystallization. If' the' rate of evaporation-dueto `boiling'exceeds` removal offthe-il water ofZ crystallizatiom` then lthe solutiontends'tobecome concentrated and must bedilutedinorder to avoid rise of its` temperature" which will increase the" vapor` pressure diierencebeyond the above indicated -rangeof 225 11". to`-230t F. which ispreferredfor producingfthefhem-ihydrate of theY type towhich the invention relatesw It' israpparen-t that the con dition which actual1y`obtains,that is, Whether the evaporationwisfle'ss than,u equals or exceeds the vamount of Water of. crystallization? removed from the gypsum; .depends'in part upon the amount' of the gypsumin'relation'to the amount ofA the-solutioniin which" at the boiling'` temperate ture of thesol-ution` theI gypsum is immersed.K It depends also upon theratel of'boilinggi. e., evaporatiomA of; ther solutiona t Itis'possiblereadily-toy control the conditions of such-*operation so as tol maintainthe requisite diferencein-the' vapor pressures of` the solution andy/of thegypsum.l If the-evaporation of the waterffrom the-solu=tion exceeds the rateofy relea'sep water-ot crystallization` from the gypsum so that the; boilingtemperature` of' the solution tends to rise abovel 230 F.; water maybe added tov maintain the'eboilingtemperature-toy vwithin the rangefbetween225" and230D F. This may be accomplished, if desired;I by returning-in whole or inf partvtojthefr-cooking vessel the vapor collected from-the-v vesselsand condensed ina suitable/condensing.apparatus.` If,on the otherhand, thewater so collectedand condensedand returnedjto they solutionl isv suiicient totend to cause the boiling temperature thereof to `drop below 225 F., some of the water may be bypassed or more calcium chloride may be added to the solution to increase the concentration and l thus restore the boiling temperature to the range and in the water, and-the idegree ofy dilution which tliewaterof crystallization asfit isrel moved may produce in the solution,. and thesize -betvveen 225 F. and 230 F. In this method of carrying out the process of the invention in boiling'fsolfutions, for example, in the range of temperatiire 225 F. to 230 F., it is merely necessary topbserve the temperature of the boiling solution in order to determine change in the con- 1l centration of the solution. Such operation at boiling temperature makes unnecessary an accurate thermostatic control such as would be required if the concentration of the salt used 12 order to prevent further dehydration to calcium sulphate anhydrite. The dried product may then be ground to the desired neness of pard ticle size.

were such that the temperature thereof might 5 T'obze IH reach or rise above that corresponding to the maximum vapor pressure diierence, namely, 1000 mm., before the solution boils. Moreover, vapor Tellltl/Sstelih l operation at boiling temperatures in solutions of fiersgge gtslisl gg suitable concentration as above described insures mm. Hg At 'resting At Pouring such a rate of conversion that the operation be- Conssncy Consistency comes commercially practical because of the provision of an adequate but not excessive vapor :12:12:: pressure difference. 32o i187 763 32 45 When any of the solutions is boiled While sub- 15 jjjjjjjjjjjjj jected to atmospheric pressure, the actual vapor 1,440 625 435 40 57 pressure of the solution at the boiling tempera- 2'780 449 320 42% 65 ture is substantially the barometric pressure at the time and location, this pressure being nor- In Table 111 are given the results of tests of many 769 mm- 0f mercury From the Curves it 20 tensile strength of the casts made of the plaster Will be found that at 225 F. the vapor pressure produced by the aboye described mei-,bod of deof gypsum in contact with the hemihydrate is bydrai-,irig gypsum by immersion in a boiling substantially 1030 mm. of mercury. At 230 F. solution of calcium Chloride, By varying the the Vapor pressure of gypsum iS Substailtially amount of salt in the solution and raising the 1150 mm. of meroury- In a boiling Solution Qf solution to the boiling temperature, for these calcium chloride under standard atmospheric tests, the process Was Carried ooi; i-,o secure depressure the vapor pressure difference at 225 F., hydration at various vapor pressure differences therefore, is 270 mmand at 230 F; is 390 mio as shown in rratio 111. It will be noted that the of mercury- TheSe VoDOI' Dressur? differences he tensile strength oi' the casts made from the plas- 1n the preferred range as above 1nd1cated. Ex- 30 ter decreases as the vapor pressure diierenee inperenee has SllOWIl that Carrying 011i? 0f the creases. The casts for these tests were made process in calcium chloride solutions with confrom siorries in which the plaster was mixed trol of the temperature 0f boiling in the Tang@ with water at testing consistency and in some between 22o F. and 2.30o F. to produce the vapor eases aiso at pouring eorisistenoyl The amount pressure dlffeleIlCeS 111 Such fange Will Secure 35 of the water used to produce these consistencies production in a commercially suitable time of in eaob ease is given in the respective columns the low water carrying product having the physifor eopsisterioy at the righi of the table cal properties which have been above described The marked difference in strength between and which is characterized by the stubby, rodthe oasi-,s made ai; 17o mm, and 2730 mm vapor like Crystals 0f the hemhydrar 40 pressure diierence is accounted for by the char- When the converslon of the gypsum lmmeised acteristic of the process that when the vapor 1n the solution to the hemihydrate 1s completed, pressure difference becomes too high the crystals by the use of suitable oppolatus, the dehydrated become of the long needlelike character and no product may be washed in boiling hot water and longer have the short rodiike form, While in the po'tlon Of the Calcium ChlOrde Solution Some cases, is when produced a rela- Clings t0 the dehydrated material may be Vapor pressure dierence for examremoved without rehydration of the hemihydrate pie 850 mm', the plaster may not have as high to calcium Sulphate dihydrate- Il? iS necessary a strength as may be secured with a lower vapor however, that the temperature Shall i? be Dei" pressure difference, the water carrying capacity mitted lto fall to any marked degree below '212 F- 50 of the plaster, Yeven with such relatively high to avoid rehydration. Since, however, 1n the yapor pressure differences, may be substantially process of the invention JBhe Vapor Pressure de' below that of plasters produced by ordinary calpressants which are used are salts which are oinirig methods, for example, the plasters proreadily soluble in water, the washing operation duced by oaioinirig in a kei-,tie Such piasiers, iS readily effeCteCi in a SllOll' time S0 that any 55 when made of the same rock of high purity as great amount of coolmg is preventedused in the tests of Table 111, ordinarily have a When the excess of the solution iS Suoenifly pouring consistency of not substantially less than removed, the hemhydrate iS dried Without Del'- '70 parts of water for 100 parts of the plaster. mittine it to cool to any temperature markedly By controlling the operation of the process so below 212 F. before removal of substantially all 60 that the vapor pressure differential does not exthe free water in order to prevent the water from ceed 1000 mm. of mercury,v water carrying care-combining to any appreciable extent with the pacities not substantially greater than 60, and in hemihydrate. This drying temperature, howthe preferred case below 50 at pouring consistever, should not be much in excess of 350 F. in ency, may be produced.

Table IV Approx. Salt Used ggaegggitil?? Cmim i Percent F.

42 230 about2hrsm. 34 47 so 23o 1% tozhrs a2 45 23o 1% to 1% hrs., 32 45 15s 23o 1% to2hrsm. a2 4s 91 234 1% hrs 35 4s Inr TableU IV are' given' the: results.-r rmade r4with different salts kused as the vaporY pressure-depressant. In each case such yav percentage of concentration of the salt infthe solution` was used that substantially all the solutions were brought to the same boiling temperature, namely, about 230 F. The amount of each salt` in percent of the weight of the water used is indicated in the second column of the table.` In the fourth column of the table is given the conversion time which, it will benoted, was about one and onefurthto two hours.. The fact that there was no great variation inthe amount of time. necessary to accomplish the conversion from the dihydrate to the hemihydrate is explained by the fact that' the boiling temperature of about 230FL', regardless ofthe salt used to produce this boiling temperature (the'quantityofV the salt being varied asabove mentioned), insured a definite vapor pressure difference. This vapor'pressure difference corresponds to the portion of Atheordinate V'at the temperature of 230 F. in the curves measured between the existing atmospheric pressure'ofabout 750 mm. and the'vapor pressure curve. A. for gypsum. This vapor pressure at 230 F. for gypsum is 1150 mm. The .vapor pressure difference, therefore, is `400 mm.v

Where the solution of the salt is a saturated solution there shouldbe a vapor pressuredifference of at least 400 mm. (or a pressure difference of at least 250 mm. lower than the'corresponding vapor .pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate. at the vcorresponding temperature). In any event the difference between the vapor pressure of the solution and the vapor pressure of thel gypsum should be maintained between 150 mm. and 1000 In this table in the fifth rcolumn is givenvthe testing consistency of the plasters which were produced by dehydration in the several solutions indicated in this table. The testing consistency for which the values are given in this table .and in Table III is dened' as:

.Testing consistency is the number of cubic centimeters of waterjwl'lich,- when mixed with 100 grams of the dry plaster in a'mixng cup,`will produce a mortar that will slump abouti/" when y the mass'is pushedv to oneY side of the ycupl with the mixing spatula and the `spatula is'then witht The corresponding pouring consistencies, measured as hereinaboveA dened for vthe pure poses of this invention, are shown inthe sixth column. It will be apparent from the table that l control of the vapor pressure difference in ac-` cordance with the invention is effective to see .cure .the low water carrying capacity which is a desired property of the calcium sulphate hemihydrate or gypsum plaster of the invention, when conversion is carried out in solutions of various acid, because of their high solubility and conse-v quent marked eiect. in depressing the `vapor pres,-V sure ofthe solutions, are preferred... rIests have indicated that the salts of` calciumotherthan calcium sulphate, whichtherefore havetafcomtmon ,metal f base with the .calcium;.sulphatanon;

' l1.4 tenir of the gfypsumr are" particularlyadvanta'e geous: because. they do not :reactwith the calcium sulphate and, therefore, thef'possibility iszmini-` mized of. forming complexsalts or reaction prod` ucts which wouldinterfere with the formation'V of the desired hemihydrate.

Contrary to the conclusions of Schoch: and Cunningham, theaprocess may; bev carried on-in a" sodium chloride. solution.` Using high purity gypsum rock ground to approximately 85,percent through a mesh sieve,the conversion has been effected in a saturated solution of sodium chloride at temperatures in the 'range of 174 F. to 211 F., these temperatures being below the boiling temperature of the solution.. After about ve and one-half hours.V heating, the 'salt isolu tion was siphoned` off from the cooking vessel. and the hemihydrate was washed. with `boiling water to removev substantially all` of thesodium chloride.. The hemihydrate was then dried at. approximately 212 F. When-.dry itwas found that the mass` of hemihydrate in thecooking Vessel was in a caked condition. Thecaked mass was broken up,y ground and screened throughaf100 mesh screen. The. resulting producthad a.. test-.- ing consistency of approximately- 33` andI had a setting timevof about '7.minutes. Theaset, dry cast was much. harder` and stronger. than a. cast made ofordinary calcined gypsum at testingcon sistency.

Consideration ofthe data presentedV inthe tables and the. discussion abovegven with respect to producingthe desiredV product by .boiling the solution, for example,l at a temperature lying. in the range between 225 and 230 F., indicates that itis possible. to producethedesired, product by means` of asolution inwater ofY any vapor pressure depressant ofthe typeabove mentioned as suitable for the invention tosecure the requisite'vapor pressure diierence by using a. greater or less amount, of a salt in.thesolutidna It is merely necessary to. add the vaporpressure de,- pressant, thatis, the salt, until the desired'boiling temperatureis reached. This will' insure thatthe vapor pressure difference, always` between barometric pressure', normally at "sealer/el' '760 mmt; at the location andr tirne'off thecperati'on and th'exvaporr pressureof the 'gypsum at the `boiling temperature `shall be'a deiinite amount'. By'se-i lecting the proper boiling temperature ofthe solution', therefore, thevvapor pressure difference may be controlled within the range which it has been discovered, according-to the invention, will produce the` hemihydrate of the desired crystal form...

Moreover, mixtures' of' different'vapor. pressure depressantsyof.l the; typezab'oveindicated'. as suitable for the-invention may be used .to produce a given boiling temperature. Indeed, it is. immaterial` that the4 proportions of vthese saltsin relation to each other shall be known or that. the total amount of either or. all. of them shall be known if sufcient amount of thewsoluble, salts are; added to` the water to securefthe boiling vtemperature to provide the requisite vapor pressure difference. Within the-scopefof the invention, therefore, any vapor` pressure. depressant of the type aboverdescribedmay beused .which is capable of substantiallyraising the boiling` temperature of'water solutionso that the vapor 'pressure diierence between the atmospheric pressure and the Vapor pressure of gypsum at the, boiling temperature will not substantially exceed 1000 mm. AsLhas been stated, .inforderrto:limit-.the

time of conversion, in general the temperatureat aciefed which the process will be carried on will be such that the vapor pressure difference will be not substantially less than 150 mm. Whileconversion to the crystals of the type desired may take place at vapor pressure differences less than 150 mm., any substantial reduction in this vapor pressure greatly increases the time of conversion so that the process becomes commercially impractical. Moreover, the pouring consistency and the other qualities of the product are not markedly improved by further reduction in the vapor pressure difference.

It is possible also to meet the condition of variation from the normal of 760 mm. in the barometric pressure which may be occasioned by elevation above sea level or other causes when carrying on the process at boiling temperatures. To do so the desired vapor pressure difference, for example 243 mm., may be selected. To the observed barometric pressure which obtains at the time and place of operation, for example 630 mm., which may be experienced at about-5000 feet elevation above sea level, is added the selected vapor pressure difference of 243 mm. This total of 873 mm. represents the vapor pressure of the gypsum corresponding to which, from the curve A, may be determined the temperature at which the solution of salt or salts must boil to give the desired vapor pressure difference of 243 mm. This temperature for these conditions is found from curve A to be about y218 F. It is only necessary, therefore, to use a solution of any of the salts in practicing the invention as above described at such concentration that the boiling temperature thereof is 218 F., in the selected example, and to carry on the process at the boiling temperature until the conversion is completed. For other barometric conditions and other desired vapor pressure diierences a similar procedure may be adopted.

The product produced by the process of the invention may have a setting time generally in the range between to 25 minutes without the addition of retarders or accelerators. This product, however, may have its setting time accelerated or retarded by the addition thereto of accelerators or retarders which are known in the art and which ordinarily are used in connection with gypsum plasters. That the product is a plaster in which substantially all of the gypsum has been converted to the hemihydrate is evidenced by the fact that under the polarizing microscope these crystals show parallel extinction characteristic of hemihydrate, whereas gypsum or calcium sulphate dihydrate shows oblique extinction. This fact, combined with the fact that when the process is properly carried out the combined Water content of the crystals is approximately 6.2% as determined by analysis, indicates that the product is hemihydrate and not a mixture of gypsum and anhydrous calcium sulphate.

The results shown were obtained by calcining small batches of rock in laboratory equipment. A manufacturing plant for the production of the product in large batches on a commercial scale has now been built and put into operation.

In the commercial operation the rock to be calcined is rst graded by screening so that all passes a 3A" square mesh screen. Each batch is calcined for about 5 hours in a boiling solution of commercial grade flake calcium chloride with the concentration of solution maintained at such degree Athat the boiling temperature is 225 F. to 228 F. l

After calcination the calclned material is washed and leached with water, dried, ground in a hammer mill, further ground in a ball mill and finally ground in a buhrstone mill. The plaster thus produced is of such iineness that approximately 99.9% passes a 100 mesh sieve, approximately 95% passes a 200 mesh sieve and approximately passes a 325 mesh sieve.

This commercial product has even lower water carrying capacity than the laboratory product and casts made from it at a given consistency, therefore, have greater strength, hardness, and density than casts made of the laboratory product at like consistency. The testing consistency of the commercial plaster is 22 to 23. Casts made at 23 Water/ plaster ratio have a dry tensile strength of about 1300 pounds per square inch, a dry compressive strength of about 15,000 pounds per square inch.

The pouring consistency of the commercial plaster is about 32 to 33. Casts made at 33 water/plaster ratio have a dry tensile strength of about 1050 pounds per square inch and a dry compressive strength of about 9,100 pounds per square inch.

Having thus described my invention, I now claim:

1. The process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate which comprises immersing the dihydrate in lump form in a solution in water of calcium chloride on the order of 30% concentration, with said lumps of such size as to pass through a l-inch screen and as to be retained on a No. 8' U. S. standard sieve, and heating said solution and dihydrate at atmospheric pressure to substantially the boiling point, such heating being continued until a substantial portion of the dihydrate is converted into hemihydrate.

2. The process of producing calcium sulphate hemihydrate K from calcium sulphate dihydrate which comprises immersing the dihydrate in lump form in a solution in water of calcium chloride on the order of 30% concentration, with said lumps of such size as to pass through a l-inch screen and as to be retained on a No. 8 U. S. standard sieve. heating said solution and dihydrate at atmospheric pressure to substantially the boiling point, such heating being continued until a substantial portion of the dihydrate is converted into hemihydrate, washing the hemihydrate with Water at a temperature not substantially below 200 F. for removing the calcium chloride, drying the washed hemihydrate by the application of heat at a temperature of not substantially less than 200 F., and then grinding the product to the desired neness.

3. Process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate which comprises immersing calcium sulphate dihydrate in a solution in water of calcium chloride at a concentration not substantially less than 15% of the calcium chloride calculated as the anhydrous salt based on the weight of the water in which said calcium chloride is dissolved, and heating said solution while subjected to atmospheric pressure to an operating temperature in the range up to and including the boiling temperature of the solution at atmospheric pressure, said operating temperature of the solution and the concentration of the solution being maintained such that the resulting vapor pressure of the solution under such conditions is lower than the vapor if?" pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at said operating temperature by a minimum amount not less than the 'pressure of a 150 mm. column of mercury and a maximum amount not greater than the pressure of a 1000 mm. column' of mer cury. such. heating being continued until 'a' substantial portion Vof the dihydrate f is converted.

into hemihydrate.

4. The process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate, which comprises immersing calcium sulphate dihydrate in a solution in water of inorganic salt material, the saturated solution of which salt material in water in the presence of calcium sulphate dihydrate will have at the boiling temperature at standard atmospheric pressure a vapor pressure at least 400 mm. of mercury lower than the corresponding vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at the corresponding temperature, and treating said solution at an operating temperature and at a concentration such that the resulting vapor pressure of the solution under such conditions is lower than the vapor pressure of calcium sulphate dihydrate in contact' with calcium sulphate hemihydrate at said operating temperature by a maximum amount not greater than the pressure of a mercury column of the height of 1000 mm., such heating of the solution and the calcium sulphate dihydrate being continued until a substantial portion of the dihydrate is converted into hemihydrate.

5. The process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate which comprises immersing calcium sulphate dihydrate in a solution in water of inorganic salt materiahthe salt material being selected from the group consisting of sulphates, nitrates and halides of the alkali and alkali earth metals selected from the group consisting of sodium, potassium, magnesium, calcium, caesium, copper and zinc and of ammonia, but restricted to those salts the saturated solutions of which in Water in the presence of calcium sulphate dihydrate will have at the boiling temperature at atmospheric pressure a vapor pressure at least 250 mm. of mercury lower than the corresponding vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at the corresponding temperature, and treating said solution at an operating temperature and at a concentration such that the resulting vapor pressure of the solution under such conditions is lower than the vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at said operating temperature by a maximum amount not greater than the pressure of a column of mercury of a height of 1000 mm., such heating of the solution and the dihydrate being continued until a substantial portion of the dihydrate is converted into hemihydrate.

6. Process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate which comprises immersing calcium sulphate dihydrate in a solution in water of inorganic salt material selected from the group consisting of sulphates, nitrates and halides of the alkali and alkali earth metals selected from the group consisting of sodium, potassium, magnesium, calcium, caesium, copper and zinc and of ammonia, but restricted to those salts the saturated'solutions of which in water in the presence of calcium sulphate dihydrate will have at the boiling temperature 181i; at atmospheric pressure a vapor pressure at least 250 mm. of mercury lowerthan the corresponding vapor pressureof calcium. sulphate dihydrate in contact with calcium sulphatehemihydrate fat:

the corresponding temperature, ,andtreating said solution while subjected to' atmospheric pressure at an operating temperature in the rangevuptor calcium sulphate hemihydrate at said operating` temperature by a minimum amount not less than the pressure of a column of mercury of a height of 1000 mm., such heating of the solution and the dihydrate being continued until a substantial portion of the dihydrate is converted into hemihydrate.

7. Process of producing a plaster from gypsum which comprises immersing calcium sulphate dihydrate in a solution in water of inorganic salt material selected from the group consisting Voi" sulphates, nitrates and halides of the alkali and alkali earth metals selected from the group consisting of sodium, potassium, magnesium, calcium, caesium, copper and zinc and of ammonia, but restricted to those salts the saturated solutions of which in water in the presence of calcium sulphate dihydrate will have at the boiling temperature at atmospheric pressure at least 250 mm. of mercury lower than the corresponding vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at the corresponding temperature, and boiling said solution while subjected to atmospheric pressure and thus driving off water of crystallization from the calcium sulphate dihydrate until the dihydrate is converted intoy the hemihydrate, said solution being at a concentration of the salt in the solution sufficient to produce a temperature substantially in the range between 225 F. and 230 F. at boiling.

8. Process of producing calcium sulphate hemihydrate from calcium sulphate dihydrate which comprises immersing calcium sulphate dihydrate in a solution in water of inorganic salt material selected from the group consisting of sulphates, nitrates and halides of the alkali and alkali earth metal-s selected from the group consisting of sodium, potassium, magnesium, calcium, caesium, copper and zinc and of ammonia, but restricted to those salts the saturated solution of which in water in the presence of calcium sulphate dihydrate will have at the boiling temperature at atmospheric pressure a vapor pressure at least 250 mm. of mercury lower than the corresponding vapor pressure of calcium sulphate dihydrate in contact with calcium sulphate hemihydrate at the corresponding temperature, said solution being maintained at such concentration that its boiling temperature at standard atmospheric pressure is between 218 F. and 248 F., and heating said solution and calcium sulphate dihydrate while subjected to atmospheric pressure to an operating temperature of at least 181 F., until a substantial portion of the dihydrate has been converted to hemihydrate.

GILBERT A. HOGGATT.

(References on following page) REFERENCES CITED OTHER REFERENCES The following references are of record n the logg'. (Hoppe-Syler) 1866, V01. 127, page me of this patent: 5 zitschr. fer Phys.. ch. Wam; Hoff) 1963, v01,

UNITED STATES PATENTS 45, page 2571 Y Number Name Date Mellor, Inorg. Kz Theo. Chem., LongHx-nans, 1,987,712 Schoch Feb. 5, 1935 Green & C0., N. Y., V01. III, 1923, pages 768'T'767. 

1. THE PROCESS OF PRODUCING CALCIUM SULPHATE HEMIHYDRATE FROM CALCIUM SULPHATE DIHYDRATE WHICH COMPRISES IMMERSING THE DIHYDRATE IN LUMP FORM IN A SOLUTION IN WATER OF CALCIUM CHLORIDE ON THE ORDER OF 30% CONCENTRATION, WITH SAID LUMPS OF SUCH SIZE AS TO PASS THROUGH A 1-INCH SCREEN AND AS TO BE RETAINED ON A NO. 8 U. S. STANDARD SIEVE, AND HEATING SAID SOLUTION AND DIHYDRATE AT ATMOSPHERIC PRESSURE TO SUBSTANTIALLY THE BOILING POINT, SUCH HEATING BEING CONTINUED UNTIL A SUBSTANTIAL PORTION OF THE DIHYDRATE IS CONVERTED INTO HEMIHYDRATE. 